Metal-organic framework (MOF) materials are a class of hybrid organic-inorganic crystalline porous materials consisting of metal ions or a cluster of metal ions connected by multi-topic organic linkers. MOF films have received great interest in the last couple of years in the field of separations, catalysis, sensing, gas storage, biomedical applications or as low-k materials in semiconductor industry, due to their tunable pore sizes and the ease of functionalization of the internal surfaces.
Understanding film processing methods such as, deposition and patterning, are key in order to further explore the potential use of these materials. Deposition of MOF films has been commonly reported to be based on growth from a solution in which the organic linker and the metal salt, providing the metal ions, are dissolved.
The deposition can take place by immersing the substrate, onto which MOF film will be grown, into a solvent-based solution containing also the dissolved organic linker and dissolved metal salt as reported by Lu and Hupp, Journal of American Chemical Society, 132(23), 7832-7833, (2010).
The deposition can also take place by immersing the substrate sequentially into a solution containing the dissolved linker and then into a solution containing the dissolved metal salt, while after each immersing step the substrate is washed with the solvent as reported by Eddaoudi et. al., Chem. Commun., 2014,50, 2089-2092.
These deposition techniques are not self-limiting, thereby requiring high number of cycles to be completed, before obtaining a homogeneous and a defect-free film. The thickness and the deposition time of films obtained through these aforementioned methods may hinder the applicability of these growth methods in various fields, where, for example, faster deposition times are desired to obtain thinner MOF films. For example, a growth rate of 100 nm/30 min is given in the work of Lu and Hupp, where they have used a methanolic solvent-based solution and zinc nitrate (Zn(NO3)2) in order to produce ZIF-8, which is a type of MOF. In the work of Eddaoudi et. al. a film thickness of 0.5 micron to 1.6 micron obtained after 150 and 300 growth cycles, where they report the use of metal ion solution and a solvent-based (CH3OH) solution also for the production of ZIF-8. Additionally, these deposition techniques involve the use of a large amount of solvent-based solution, which is usually toxic. Furthermore, they involve the use of chloride and nitrate salts, which are commonly used as sources of metal ions, which are not compatible with industrial requirements due to corrosion and safety hazards.
The problems associated with the use of a solvent-based solution and the use of metal salts have been overcome in the art by producing MOF films from metal oxides or hydroxides. For example, in the work of Stassen et. al. (Cryst. Eng. Comm., 15, 9308-9311, 2013), formation of ZIF-8,which is a type of MOF film, is reported in the absence of a solvent-based solution. Starting ZnO is sputtered on a surface. However, the organic linker, which is 2-methylimidazole, is applied as a powder over the surface containing the metal oxide in this technique. This can pose a limitation to the use of this technique in certain industries, for example semiconductor industry, due to the fact that the use of powders is strictly forbidden within the manufacturing environment.
There is, therefore, a need in the art, to produce thin MOF films using a method that is applicable to a wider range of industries.